Effect of Iron Substitution on the Structural, Electric, and Magnetic Properties inR-Type PbFexV6−xO11, a Frustrated System

Single-phase material of the solid solution PbFe_xV_6−xO₁₁was prepared by solid-state reaction over the range 1≤x≤1.75. Powder preparations of PbV₆O₁₁, however, were always accompanied by a small amount of impurity. The noncentrosymmetric space groupP6₃mcof PbV₆O₁₁, which has anR-block structure, remains unchanged with iron substitution. The crystal structure of PbFe_1.75V_4.25O₁₁was refined from single-crystal X-ray data toR=0.049. The iron atoms are located preferentially in specific crystallographic sites. The triangular–bipyramidal site is completely occupied by Fe³⁺while the interstices of the central octahedral layer are solely occupied by the vanadium V(1) atoms. The other two crystallographic sites have mixed Fe/V occupancies. Magnetic measurements performed on nearly single-phase PbV₆O₁₁and the iron substituted phases reveal a spin-glass-type behavior with freezing temperaturesT_fof 50 and 65 K, respectively.T_fis independent of the iron ratio. This spin-disordered system is attributed to strong frustration occuring in thea–bplane because of the existence of V(1) trimers providing alternatively long and short V–V distances along theaaxis. Resistance and Seebeck coefficient measurements indicatep-type conduction aboveT_fwhile the system becomes an insulator belowT_f. Fe³⁺ions do not participate in the hopping process, and they block conduction parallel to thecaxis because of their location in the bridging dimeric octahedral and bipyramidal sites.     

A new paradigm for macromolecular crystallography beamlines derived from high-pressure methodology and results

Biological structures can now be investigated at high resolution by high-pressure X-ray macromolecular crystallography (HPMX). The number of HPMX studies is growing, with applications to polynucleotides, monomeric and multimeric proteins, complex assemblies and even a virus capsid. Investigations of the effects of pressure perturbation have encompassed elastic compression of the native state, study of proteins from extremophiles and trapping of higher-energy conformers that are often of biological interest; measurements of the compressibility of crystals and macromolecules were also performed. HPMX results were an incentive to investigate short and ultra-short wavelengths for standard biocrystallography. On cryocooled lysozyme crystals it was found that the data collection efficiency using 33 keV photons is increased with respect to 18 keV photons. This conclusion was extended from 33 keV down to 6.5 keV by exploiting previously published data. To be fully exploited, the potential of higher-energy photons requires detectors with a good efficiency. Accordingly, a new paradigm for MX beamlines was suggested, using conventional short and ultra-short wavelengths, aiming at the collection of very high accuracy data on crystals under standard conditions or under high pressure. The main elements of such beamlines are outlined.     

Singularities of the solution of a mixed problem for a general second order elliptic equation and boundary stabilizationof the wave equation

We deal here with a second order elliptic mixed problem which is posed in a regular open bounded domain of . We study the regularity of its solution. We apply our results to the boundary stabilization of the wave equation.